With reference to FIG. 1, a conventional high-voltage charge pump circuit 10, such as a Dickson charge pump circuit, typically provides an output voltage Vout with two voltage levels for operation of a non-volatile memory. One output voltage is a low-voltage Vdd supply voltage that is used for read operations of, for example, a non-volatile memory load 12. For erasing and programming operations of the non-volatile memory load 12, a high-voltage Vpp voltage is required. The high-voltage Vpp voltage is used in the non-volatile memory load 12 for high-voltage drivers, latches, x-decoders, y-decoders, etc.
One method of regulating the high-voltage output of the charge pump circuit 10 uses a diode clamp circuit 14 to hold the high voltage Vout voltage at a fixed value and to absorb excess current as T_clamp in the diode clamp circuit 14, while the high-voltage charge pump circuit 10 delivers an output current I_out and the load 12 takes a load current I_load.
FIG. 2 illustrates a typical series Zener diode clamp circuit 20 that is connected between a Vout terminal 22 of a charge pump and a ground reference terminal 24. The output voltage and current of a typical charge pump circuit are inversely related, such that an increase in output current produces a decrease in output voltage and a decrease in output current produces an increase in output voltage. To provide a fixed output voltage, a Zener diode clamp circuit requires the charge pump circuit to produce excess output current. First and second reverse biased Zener diodes 26, 28 each have reverse-bias voltage drops of 7 volts and first and second forward biased Zener diodes 30, 32 each have forward-bias voltage drops of 0.75 volts. The total voltage drop for the Zener diode clamp circuit 20 is 15.5 volts, which clamps the Vout voltage at a Vpp voltage level of 15.5 volts. This type of high-voltage charge pump circuit 10 with a Zener diode clamp circuit 20 provides a passive technique for regulating the Vout voltage at a high-voltage Vpp level. For practical circuits, the Vout voltage level varies somewhat as a function of the load current. This type of regulation scheme has the charge pump circuit producing excess current that is dumped in the diode clamp circuit 14 to maintain Vout at a substantially fixed Vout level.